Shrouds employed in a gas turbine surround and in part define the hot gas path through the turbine. Shrouds are typically characterized by a plurality of circumferentially extending shroud segments arranged about the hot gas path, with each segment including discrete inner and outer shroud bodies. Conventionally, there are two or three inner shroud segments for each outer shroud segment, with the outer shroud segments being secured to the stationary inner shell or casing of the turbine and the inner shroud segments being secured to the outer shroud segments. The inner shroud segments directly surround the rotating parts of the turbine, i.e., the rotor wheels carrying rows of buckets or blades.
Because the inner shroud segments are exposed to hot combustion gases in the hot gas path, systems for cooling the inner shroud segments are oftentimes necessary to reduce the temperature of the segments. This is especially true for inner shroud segments in the first and second stages of a turbine that are exposed to very high temperatures of the combustion gases due to their close proximity to the turbine combustors. Heat transfer coefficients are also very high due to rotation of the turbine buckets or blades.
To cool the shrouds, typically relatively cold air from the turbine compressor is supplied via convection cooling holes that extend through the segments and into the gaps between the segments to cool the sides of the segments and to prevent hot path gas ingestion into the gaps. The area that is purged and cooled by a single cooling hole is small, however, because the velocity of the cooling air exiting the cooling hole is high and the cooling air diffuses into the hot gas flow path.
Typically, the post-impingement air leaks into the gas path between two inner shrouds, through hard/cloth seals located on the seal slot surface. Shroud slash faces, in particular, above the bucket region, are the life-limiting regions, mainly due to oxidation. This is caused by the continuous ingestion of hot gases thrown by the bucket towards the shroud inter-segment gaps. Traditional cooling methods use cooling holes along the slash face drilled from post-impingement cold section, or discrete perpendicular channels machined along the length of the seal slot, which improves the slash face cooling to certain extent, but whose effects are very localized as they do not cover the entire length of low-life slash face region.
Another component of gas turbines that includes seal slots are nozzles. A nozzle may be formed by a plurality of sections, or segments, and seals between adjacent segments. Service run nozzles in a gas turbine may have distorted sidewalls as a result of previous weld repairs or due to stress relief during service. Creep strain due to applied loads at operating temperatures may also contribute to distortion. This movement of the sidewalls will cause the seal slots that are contained within the sidewalls to be out of position relative to engine center.
If the sidewalls are not pressed back into position, the seal slots between adjacent segments would not be aligned with each other, and it may prove impossible to fit the seals in place. Alternatively, it may be possible to force the seals into the slots but this would lock the nozzle segments together such that they could not move or “float” relative to each other. This float is necessary to allow for thermal expansion and to ensure the segments load up against the sealing faces (hook fit and chordal hinge) during operation. If they are locked together, it is likely they will be skewed and will not load against their sealing faces. This will result in compressor discharge air leaking directly into the hot gas path and will reduce the amount of air available for combustion and for cooling of the nozzle. The result of reduced air for combustion will be lower performance of the turbine and increased emissions. A reduction in available cooling air will result in increased oxidation of the nozzle due to a resultant higher metal temperature and the lack of cooling will also cause changes to thermal gradients within the nozzle leading to increased cracking of the part. This will increase subsequent repair costs and may reduce the life of the parts.
Misaligned sidewalls may also result in flow path steps. The hot gas will not have a smooth path but will be tripped by the mismatch between adjacent nozzle segments, resulting in turbulent flow and reduced energy of the gas stream, thereby reducing performance. Turbulent flow also increases thermal transfer to the nozzle and so will raise the metal temperature, leading to increased oxidation and cracking.